One mystery solves another: Fast radio bursts detect missing matter
Only about five percent of the content of the universe is made up of regular (or baryonic) matter – and we still don’t know where most of that is. Now, an international team of astronomers has developed a creative new method to detect this missing matter, using the equally-mysterious fast radio bursts (FRBs).
We have some idea of how much matter should have been created in the Big Bang – and when we look at the stars, planets, galaxies, black holes and everything else today, there just doesn’t seem to be enough. Observations of very distant objects show us what the universe looked like billions of years ago, and after a certain point in time a huge amount of this matter just disappears.
Around 40 percent of the normal matter budget is missing in the universe today. But that’s not to say that we don’t have any idea where it is – astronomers have long hypothesized that it’s floating around in intergalactic space. It takes the form of a gas that’s so extremely diffuse that there’s only about one electron per cubic meter – for reference, regular old air has about an octillion electrons per cubic meter.
That, of course, makes it very hard to detect. But the new study, involving researchers from the International Centre for Radio Astronomy Research (ICRAR), Curtin University, CSIRO, Swinburne University of Technology, and Macquarie University, may have found a way.
Intriguingly, the key to unraveling one mystery is to use another. Fast radio bursts are extremely energetic flashes of radio signals that regularly pour in from all corners of the sky, each lasting mere milliseconds. Astronomers don’t know what causes them, but the leading theory is that they come from pulsars or magnetars.
Whatever they are, the team put them to good use on this study. The astronomers studied five FRBs detected by ASKAP in Western Australia, to get a sense of what the signals passed through on their incredibly long journeys to Earth.
If they pass through completely empty space, then all wavelengths of the signal would arrive at the same time. But that wasn’t the case – the team found that certain wavelengths arrived on a delay. That indicates that the signal interacted with molecules along the way.
Next, the researchers can use optical measurements to check how far away those FRBs were. Then, the density of the missing matter can be calculated from the distance to the FRB and the delay between the wavelengths. By extrapolating from that, the team says that the cosmic baryon density comes to account for the amount that was predicted to be missing.
It's important to note, however, that this isn't dark matter. That's an entirely separate mystery regarding a form of matter unlike anything we've ever seen. This newly-detected "missing" matter is just the regular old stuff that we ourselves are made of.
This isn’t the first hunt for missing matter either. Other astronomers claim to have closed the gap using similar methods – in one case, ESA’s XMM-Newton observatory performed similar experiments using X-ray signals from distant quasars.
But while this missing matter has been detected, the researchers say that it hasn’t exactly been “found” yet. They can’t be sure where exactly it is, or in what amounts, or even what form it takes. These answers could be discovered by studying more FRBs in future. The team already has hopes to expand the five used in this study up to 100, to paint a clearer picture.
The research was published in the journal Nature. The work is described in the video below.